Expanded packing products, which are resilient polymeric foams, enjoy widespread use as packing materials. Of these, polystyrene has been predominant, not only because of its strength and stability, but in large part because of its very low cost. However, polystyrene foams have the serious disadvantage that they are not degradable under the environmental conditions that prevail in compost heaps, landfills and other common disposal routes for such materials.
As a result of increasing concerns with the ecological impact of disposing of such packing materials, particularly the utilization of landfills, considerable attention has been paid in recent years to the use of starch-polymer mixtures as a substitute for synthetic polymers in polymeric foam packing materials. Starches and many of their derivatives, of course, have the advantage that they are biodegradable. Polymers are used in a minor amount with the starch to increase stability, stiffness, elasticity, etc., of the packing foam. However, the amount of synthetic polymer, even if it is not itself degradable, is sufficiently low so that it is does not interfere substantially with the degradability of the starch.
In the manufacture of foams for packing applications using starch as the polymer base, the starch is admixed with a minor amount of synthetic polymer and water to form an aqueous starch-polymer powder mixture, which is then extruded to form various shapes such as spheres, pellets or "peanuts."
The manufacture of extruded foam shapes in the above-described manner has proved to be troublesome because it has been found to be difficult to achieve uniform moisture content, which is vital to the method, and also to achieve consistent feed properties. In addition, the extruder feed made in this manner tends to plug the extruder, which necessitates shutting down and cleaning the unit before production can be resumed.
The equipment for this mode of operation is expensive and requires a higher level of operating skill to achieve reliability. Furthermore, the process is energy intensive and requires an extra operational step (drying).
In a variation of this process, starch and polymer are heated and mixed prior to the addition of water. The still dry admixture is then mixed with water and degassed under vacuum, after which it is fed continuously into a single or twin screw extruder to form pellets. The material is shipped in pellet form. The loose fill manufacturer then feeds the pellets to a second extruder in which the pellets are remelted and extruded into the various product shapes. (See Werner and Pfleiderer, GmbH brochure No. 05 140/1-2.0-X.92 KODO, entitled "Processing: Plastics Recycling.")
It has been found that this second method overcomes most of the disadvantages of the first-described method. However, it is more expensive both because of the increased number of steps in the method and in the higher cost of the equipment. Therefore, there is considerable need for a method of producing such foamed starch-polymer mixtures which is both easier and less expensive to operate.